Scanning electron microscopes often have the capability to perform electron microprobe measurements on various materials. This involves the use of energy dispersive spectrometry (EDS) to measure the energy and intensity of characteristic X-ray fluorescence lines emitted from various elements in a sample, as a result of being probed and excited by an electron beam. The elemental composition can then be determined as a function of location on the sample. Present EDS systems offer limited energy resolution, which limits its ability to measure compositions accurately, as well as detecting trace elements. Wavelength dispersive spectrometers (WDS) offer good energy resolution but data collection is very time intensive. The present invention, a monolithically fabricated X-ray thermal (bolometer, calorimeter) array combined with a means of cooling to cryogenic temperatures, offers significantly improved performance and is presented as a replacement for conventional detection systems.
An X-ray thermal detector is comprised of two parts placed in good thermal contact; an X-ray absorbing material, and a thermometer material. An X-ray photon is absorbed in the X-ray absorbing layer, creating electron-hole pairs in this material. These electron-hole pairs recombine and convert their energy to heat. The heat, which is proportional to the energy of the original photon, is conducted through the absorbing material to the thermometer. The thermometer's temperature is raised in response to this induced heat, and its resistance or other electronic properties is changed. These changes are detected electronically and amplified and filtered to produce a peak voltage. The peak voltage corresponds to one X-ray photon count; the magnitude of the peak voltage is a measure of the photon energy. After many photons are counted, their numbers as a function of energy are registered in a multichannel analyzer, and a plot of photon count versus photon energy is generated. Peaks in this energy distribution plot correspond to characteristic X-ray fluorescence lines of various elements present in the material emitting these X-rays. The intensity of the lines (peaks) is a measure of the relative amounts of the elements present.
An advantage of thermal detectors is the extremely high energy resolution obtainable when cooled to low temperatures, typically near 0.1K, in order to reduce the total heat capacity of the detector. Conventional solid state detectors (lithium-drifted silicon detectors) have energy resolutions around 160 eV, while the natural line width of characteristic fluorescence line is of the order of a few eV. For multi-element samples, the 160 eV broadening of the fluorescence lines may result in line overlap. This results in reduced sample compositional accuracy, and small peaks from elements of low concentration may be obscured entirely. Another disadvantage of poor energy resolution is high trace element detection limits. Conventional solid state detectors have a broad energy window which leads to a high background count rate and obscures the weak fluorescence line being measured. The thermal detector, on the other hand, can have an energy resolution on the order of a few eV, so peak overlap is greatly reduced, and the reduced background from its narrow energy window significantly lowers the minimum concentration of trace material that can be detected.